System and methods for time and frequency division passive optical network

ABSTRACT

A coherent passive optical network includes a downstream transceiver and first and second upstream transceivers in communication with an optical transport medium. The downstream transceiver includes a downstream processor for mapping a downstream data stream to a plurality of sub-bands, and a downstream transmitter for transmitting a downstream optical signal modulated with the plurality of sub-bands. The first upstream transceiver includes a first local oscillator (LO) for tuning a first LO center frequency to a first sub-band of the plurality of sub-bands, and a first downstream receiver for coherently detecting the downstream optical signal within the first sub-band. The second upstream transceiver includes a second downstream receiver configured for coherently detecting the downstream optical signal within a second sub-band of the plurality of sub-bands. The downstream processor dynamically allocates the first and second sub-bands to the first and second transceivers in the time and frequency domains.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application Ser. No. 62/849,737, filed May 17, 2019, which isincorporated herein by reference in its entirety.

BACKGROUND

The field of the disclosure relates generally to fiber communicationnetworks, and more particularly, to access networks for transmittingcoherent optical signals.

Bandwidth requirements in optical access networks have growntremendously in recent years, due to rapidly increasing demand from newbusiness and technology drivers, such as mobile Internet, 5G, cloudcomputing and networking, high bandwidth video streaming services, “bigdata,” social media, Internet of Things (IoT), and mobile data delivery.In conventional access networks, the passive optical network (PON) hasbecome a dominant architecture to meet such end user high capacitydemand in point-to-multipoint (P2MP) systems, as evidenced by therecently-developing standards in next generation (NG) high speedtime-division multiplexing PON (TDM-PON) standards.

To further improve the data rate and bandwidth capacity of the accessnetwork, coherent TDM-PON and coherent wavelength-division-multiplexing(WDM)-PON/ultra-dense WDM-PON solutions have been proposed and reportedfor NG access networks. In particular, recent high-speed (i.e., greaterthan a 100G data rate) TDM-PON coherent detection proposals offerhigh-speed data transmission capability with advanced modulationformats, while also enhancing the link power budget due to the increasedsensitivity of the coherent technology. Coherent detection technology inthe PON paradigm demonstrate improved receiver sensitivity throughcoherent beating of the transmission signal with a clean localoscillator (LO) signal. In comparison with WDM-PON, high-speed coherentTDM-PON solutions based on a single wavelength achieve betterstatistical multiplexing for bandwidth sharing and transparent signaltransmission with colorless components, which results in a simplifieddeployment, lowered operating expense (OPEX), and saved wavelengthresources.

A typical conventional coherent detection-based TDM PON system, however,requires symmetrical hardware complexity at both the central office(e.g., an Optical Line Terminal (OLT)) and the end-device of the enduser (e.g., an Optical Network Unit (ONU)) to achieve this statisticalmultiplexing gain. That is, the conventional ONUs must have the sametransceiver bandwidth as the conventional OLT. For example, aconventional 100G TDM-PON system requires a 100G transmitter at theOLT-side and a 100G receiver at the ONU-side for downstream transmissionfrom the OLT to the ONU. If the system supports coherent 100Gtransmission in the downstream and the upstream directions, then asymmetrical 100/100G PON requires 100G coherent transceivers (Co-TRx) atboth the OLT and ONU side. A conventional symmetrical 100/100G TDM-PONsystem is described further below with respect to FIG. 1.

FIG. 1 is a schematic illustration of a conventional TDM-PON system 100.In the example depicted in FIG. 1, system 100 is a conventional coherentTDM-PON having symmetrical 100G capacity. System 100 includes acentralized OLT 102, a splitter 104 (e.g., a passive or powersplitter/combiner), and a plurality of ONUs 106 (four ONUs 106(1-4), inthis example), which may be further in communication with a plurality ofusers or customer premises (not shown). OLT 102 is in communication withsplitter 104 over one or more optical fibers 108, and is typicallylocated within a central office, a communications hub, or a headend ofan optical link (not shown), and functions to convert standard signalsfrom a service provider (not shown) to the frequency and framing used bysystem 100, and also for coordinating multiplexing between conversiondevices on ONUs 106. Because system 100 is a symmetrical 100/100G PON,each ONU 106 is configured for 100G Co-TRx capability, and OLT 102includes at least one 100G OLT coherent transceiver 110. System 100though, as a coherent PON working only in TDM mode, is challenged by twoparticular disadvantages: (1) limited resource allocation capability;and (2) considerably high cost for implementation.

With respect to resource allocation, because the TDM-PON only supportstime domain bandwidth allocation, the flexibility of resource allocationis limited. Therefore, it has been desirable to be able to divide thetotal bandwidth into multiple sub-bands in order for the system tosupport more flexible bandwidth allocation, such as from WDM techniques.However, full WDM suffers colorful components in distribution networksand lacks statistical multiplexing gain. A hybrid combination of TDM,WDM, and TWDM techniques have been recently introduced in an NG-PON2system. This hybrid system, however, still requires colorful components(i.e., similar to full WDM), and therefore multiple lasers sources andmultiple transceivers to achieve bandwidth bundling and sharing over10G. This hybrid PON thus adds additional, and undesirable, complexityto both the ONU and OLT. Accordingly, there is a need for improved PONarchitectures and techniques that efficiently utilize the advantages oftime and frequency division multiplexing for bandwidth allocation, butwithout suffering the disadvantages of either technique.

With respect to the system cost, a 100G coherent transceiver isconsidered to be a very expensive hardware component at the ONU-side,particularly where an end-user desires only relatively small bandwidthor low bandwidth tier services. The conventional symmetrical 100/100GPON configuration, however, still requires a 100G transceiver for eachend user sharing the total 100G bandwidth with other users. Thus, thereis no cost difference for end-user devices among the multiple differentsubscribers; the cost of the ONU for the lowest-tier service subscriberis the same as the cost to the highest-tier subscribers, irrespective ofthe different services. For example, a 2.5G, 5G, or 10G bandwidthsubscriber to the 100/100G PON still requires a 100G transceiver ONU toshare the total 100G bandwidth.

Accordingly, given that the PON paradigm is very sensitive to cost, andthat the cost of the ONUs in a P2MP PON system dominates the totalhardware cost, there is also a need for lower cost solutions that enableflexible bandwidth sharing.

SUMMARY

In an embodiment, a coherent passive optical network (PON) includes adownstream transceiver, first and second upstream transceivers, and anoptical transport medium in operable communication with the downstreamtransceiver and first and second upstream transceivers. The downstreamtransceiver includes a downstream processor for mapping a downstreamdata stream to a plurality of sub-bands, and a downstream transmitterfor transmitting a downstream optical signal modulated with theplurality of sub-bands. The first upstream transceiver includes a firstlocal oscillator (LO) for tuning a first LO center frequency to a firstsub-band of the plurality of sub-bands, and a first downstream receiverfor coherently detecting the downstream optical signal within the firstsub-band. The second upstream transceiver includes a second downstreamreceiver configured for coherently detecting the downstream opticalsignal within a second sub-band of the plurality of sub-bands. Thedownstream processor dynamically allocates the first and secondsub-bands to the first and second transceivers in both of the time andfrequency domains.

In an embodiment, a coherent optical transmitter includes a lasersource, a data source having a data stream, a processor, and amodulator. The processor includes a serial-to-parallel converter forconverting the data stream into a plurality of parallel data streams, amodulation mapping unit configured to code and map each of the pluralityof parallel data streams to a constellation of a modulation format togenerate a plurality of modulated data streams, a sub-band mapping unitconfigured to map each of the plurality of modulated data streams to arespective sub-band modulation spectrum from a plurality of frequencysub-bands to generate a plurality of sub-band data signals, and adigital-to-analog converter configured to combine the plurality ofsub-band data signals and generate a combined analog sub-band output ofprocessed data. The modulator is configured to modulate the combinedanalog sub-band output of processed data with the laser source togenerate a modulated optical signal for transmission over an opticaltransport medium.

BRIEF DESCRIPTION

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of a conventional time-divisionmultiplexing passive optical network system.

FIG. 2 is a schematic illustration of an exemplary coherent time andfrequency division multiplexing passive optical network system.

FIG. 3 depicts an allocation scheme for frequency resources and localoscillators in the system depicted in FIG. 2.

FIG. 4 is a schematic illustration of a modulation scheme for atransmitter of the optical line terminal depicted in FIG. 2.

FIG. 5 is a schematic illustration of a single band detection and signalrecovery scheme for a receiver of the optical network units depicted inFIG. 2.

FIG. 6 is a schematic illustration of a multi-band detection and signalrecovery scheme for a receiver of the optical network units depicted inFIG. 2.

FIG. 7 is a schematic illustration of a single sub-band modulationscheme for a transmitter of the optical network units depicted in FIG.2.

FIG. 8 is a schematic illustration of an exemplary simulation model of acoherent time and frequency division multiplexing passive opticalnetwork.

FIG. 9 depicts an allocation scheme for frequency resources and localoscillators of the simulation model depicted in FIG. 8.

FIG. 10 is a graphical illustration depicting a comparative results plotof downstream receiver sensitivity for the optical network unitsdepicted in FIG. 8.

FIGS. 11A-C are graphical illustrations depicting detected signalspectrum plots for the optical network units depicted in FIG. 8.

FIG. 12 is a graphical illustration depicting a comparative results plotof receiver sensitivity for upstream detected signals from the opticalnetwork units depicted in FIG. 7.

FIGS. 13A-C are graphical illustrations depicting respective detectedsignal spectrum plots for the upstream detected signals depicted in FIG.11.

Unless otherwise indicated, the drawings provided herein are meant toillustrate features of embodiments of this disclosure. These featuresare believed to be applicable in a wide variety of systems including oneor more embodiments of this disclosure. As such, the drawings are notmeant to include all conventional features known by those of ordinaryskill in the art to be required for the practice of the embodimentsdisclosed herein.

DETAILED DESCRIPTION

In the following specification and the claims, reference will be made toa number of terms, which shall be defined to have the followingmeanings.

The singular forms “a,” “an,” and “the” include plural references unlessthe context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately,” and “substantially,” are notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged; such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.

As used herein, the terms “processor” and “computer” and related terms,e.g., “processing device”, “computing device”, and “controller” are notlimited to just those integrated circuits referred to in the art as acomputer, but broadly refers to a microcontroller, a microcomputer, aprogrammable logic controller (PLC), an application specific integratedcircuit (ASIC), and other programmable circuits, and these terms areused interchangeably herein. In the embodiments described herein, memorymay include, but is not limited to, a computer-readable medium, such asa random access memory (RAM), and a computer-readable non-volatilemedium, such as flash memory. Alternatively, a floppy disk, a compactdisc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or adigital versatile disc (DVD) may also be used. Also, in the embodimentsdescribed herein, additional input channels may be, but are not limitedto, computer peripherals associated with an operator interface such as amouse and a keyboard. Alternatively, other computer peripherals may alsobe used that may include, for example, but not be limited to, a scanner.Furthermore, in the exemplary embodiment, additional output channels mayinclude, but not be limited to, an operator interface monitor.

Further, as used herein, the terms “software” and “firmware” areinterchangeable, and include computer program storage in memory forexecution by personal computers, workstations, clients, and servers.

As used herein, the term “non-transitory computer-readable media” isintended to be representative of any tangible computer-based deviceimplemented in any method or technology for short-term and long-termstorage of information, such as, computer-readable instructions, datastructures, program modules and sub-modules, or other data in anydevice. Therefore, the methods described herein may be encoded asexecutable instructions embodied in a tangible, non-transitory, computerreadable medium, including, without limitation, a storage device and amemory device. Such instructions, when executed by a processor, causethe processor to perform at least a portion of the methods describedherein. Moreover, as used herein, the term “non-transitorycomputer-readable media” includes all tangible, computer-readable media,including, without limitation, non-transitory computer storage devices,including, without limitation, volatile and nonvolatile media, andremovable and non-removable media such as a firmware, physical andvirtual storage, CD-ROMs, DVDs, and any other digital source such as anetwork or the Internet, as well as yet to be developed digital means,with the sole exception being a transitory, propagating signal.

Furthermore, as used herein, the term “real-time” refers to at least oneof the time of occurrence of the associated events, the time ofmeasurement and collection of predetermined data, the time for acomputing device (e.g., a processor) to process the data, and the timeof a system response to the events and the environment. In theembodiments described herein, these activities and events occursubstantially instantaneously.

As used herein, “modem termination system” (MTS) refers to a terminationunit including one or more of an Optical Network Terminal (ONT), anoptical line termination (OLT), a network termination unit, a satellitetermination unit, a cable modem termination system (CMTS), and/or othertermination systems which may be individually or collectively referredto as an MTS.

As used herein, “modem” refers to a modem device, including one or morea cable modem (CM), a satellite modem, an optical network unit (ONU), aDSL unit, etc., which may be individually or collectively referred to asmodems.

As described herein, a “PON” generally refers to a passive opticalnetwork or system having components labeled according to known namingconventions of similar elements that are used in conventional PONsystems. For example, an OLT may be implemented at an aggregation point,such as a headend/hub, and multiple ONUs may be disposed and operable ata plurality of end user, customer premises, or subscriber locations.Accordingly, an “uplink transmission” refers to an upstream transmissionfrom an end user to a headend/hub, and a “downlink transmission” refersto a downstream transmission from a headend/hub to the end user, whichmay be presumed to be generally broadcasting continuously (unless in apower saving mode, or the like).

Exemplary systems and methods for coherent PON solutions are describedherein. In an exemplary embodiment, an innovative coherent PON solutioneffectively implements coherent time and frequency division multiplexingTFDM for the exemplary PON (coherent TFDM-PON) based on multiplexing ofsub-bands. The present coherent TFDM-PON supports asymmetric ONU/OLThardware configurations, and also pay-as-you-go ONU costs based on therelevant bandwidth subscription, which represent significantimprovements over conventional TDM and WDM PONs, as well as recenthybrids thereof.

The present embodiments differ from conventional WDM-PONs and TWDM-PONs,in that the OLT and ONUs of the TFDM-PON described herein may beconfigured to operate at the same wavelength grid with a small frequencytuning. The systems and methods herein advantageously utilize wavelengthselection capabilities of coherent detection to eliminate the need foroptical filtering or wavelength selective components for frequencyselection. The coherent TFDM-PON systems and methods described hereinsupport more flexible bandwidth sharing in both the time and frequencydomains, and therefore realize significant advantages over conventionaltechniques for flexibly assigning which user occupies which channelsutilizing coherent optics tunability and frequency selectivity.

The present coherent TFDM-PON is further advantageous over conventionalPON systems in that it is fully compatible with the widely deployedconventional TDM-PON, and also will work using a power splitter-basedoptical distribution network (ODN) without need for colorful components.A TFDM-PON according to the present embodiments need only onetransceiver at the OLT-side for a transceiver at the ONU-side and, byusing frequency division multiplexing, the overall scheduling latency inthe coherent TFDM-PON system is significantly lower than in the TDM-PON.An exemplary coherent TFDM-PON is described further below with respectto FIG. 2.

FIG. 2 is a schematic illustration of an exemplary coherent TFDM-PONsystem 200. Coherent TFDM-PON system 200 is similar to TDM-PON system100, FIG. 1, in general architectural configuration, and includes acentral OLT transceiver 202, a splitter 204, and a plurality of ONUs 206(1-n ONUs 106(1-n), where n=4 in this example). System 200 differs,however, from system 100, in that each of ONUs 206 is of a differenttype from one another. That is, in this example, ONU 206(1) is depictedas a 25G transceiver, ONU 206(2) as a 50G transceiver, ONU 206(3) as a75G transceiver, and ONU 206(n) as a 100G transceiver.

A system according to the embodiment depicted in FIG. 2 isadvantageously capable to significantly reduce the overall cost of thecoherent PON by utilizing different types of ONUs among the plurality ofend users in the network. In this example, system 200 is a 100G coherentPON, and in exemplary operation, OLT 202 is in communication withsplitter 204 over an optical transport medium 208 (e.g., a single modefiber (SMF)), and transmits a 100G downstream (DS) signal 210, which maybe modulated with N sub-bands 212 (4 sub-bands illustrated in FIG. 2)each having a bandwidth equal to 1/N of the total bandwidth. At theONU-side, each of ONUs 206 may detect one or more of sub-bands 212depending on the receiver bandwidth of that particular ONU transceiver206. For example, a low-bandwidth tier subscriber may be able to detectonly one of the N sub-bands 212. Subscribers having larger bandwidth ONUreceivers would be more likely able to detect multiple sub-bands 212.Upstream (US) transmission from transmitters (not separately shown) ofONUs 206 may operate similarly.

According to the exemplary embodiment depicted in FIG. 2, downstreamsignal 210 has a total bandwidth of 100G, which is divided into foursub-bands 212. The four ONUs 206, of different ONU types, receivesub-bands 212 the hardware bandwidth of the respective ONU receiver. Forexample, a 25G ONU (e.g., ONU 206(1)) may have a transceiver supportinga 25 Gbps data rate, and the required bandwidth for 25G ONU transceiversis only 6.25 GHz. A 50G ONU (e.g., ONU 206(1)), on the other hand, wouldrequire a 12.5 GHz bandwidth, whereas a 100G ONU (e.g., ONU 206(4))would require a 25 GHz bandwidth.

Exemplary architectures of coherent PON architectures, as well as therespective components thereof, are described in greater detail in U.S.Pat. Nos. 9,912,409, 10,200,123, and co-pending U.S. patent applicationSer. No. 15/609,461, filed May 31, 2017, to the present inventors, thedisclosures of all of which are incorporated by reference herein.

FIG. 3 depicts an allocation scheme 300 for frequency resources and LOsin system 200, FIG. 2. In an exemplary embodiment, scheme 300illustrates the frequency resource and corresponding LO allocations inthe 100G coherent TFDM-PON of system 200 with respect to sub-bands 212(i.e., four adjacent DS sub-bands 212(1), 212(2), 212(3), 212(4)). Forease of illustration, the “0” frequency represents the base frequencycorresponding to the wavelength grid for the downstream or upstreamtransmissions, and the coherent TFDM-PON is capable of supporting 25G,50G, 75G and 100G ONUs (i.e., ONUs 206(1), 206(2), 206(3), and 206(4),respectively).

Accordingly, in this example, the low-speed 25G ONU case (i.e., ONU206(1)) may utilize any of the four sub-bands 212(1-4) as the frequencyresource to allocate. The respective LO and carrier may thus be tuned tothe center frequency of the particular sub-band 212 utilized, therebyproviding four different allocation options for the 25G ONU in scheme300. Similarly, for the 50G ONU case (i.e., ONU 206(2)), two adjacentsub-bands 212 may be used for resource allocation, thereby providingthree different allocation options for the 50G ONU (e.g., sub-bands212(1-2), sub-bands 212(2-3), or sub-bands 212(3-4)). In contrast, the75G ONU case (i.e., ONU 206(3)) provides two resource allocation options(e.g., sub-bands 212(1-3) or sub-bands 212(2-4)), whereas the 100G ONUcase (i.e., ONU 206(4)) provides only one resource allocation option(e.g., sub-bands 212(1-4)), since all four sub-bands 212 are occupied.

System 200 and scheme 300 are described with respect to four sub-bands212, but this number is provided for purposes of illustration, and notin a limiting sense. The person of ordinary skill in the art willappreciate, after reading and comprehending the present application,that the principles described herein are applicable to coherent TFDM-PONsystems utilizing any number of sub-bands. Additionally, although thenumber of sub-bands may be any number, in practice, it may be desirableto balance the flexibility of frequency multiplexing with the additionalcomplexity caused by fine divisions in frequency bands. For example, asexplained further below with respect to FIG. 7, in some cases, there maybe only two sub-bands to support two different types of ONU (e.g., 50Gand 100G ONUs).

The particular sub-band division schemes depicted in FIGS. 2 and 3 arealso provided for illustrative purposes, and not in a limiting sense.The systems and methods described herein are also applicable to otherpossible frequency sub-band division schemes. Furthermore, although thepresent embodiments are particularly advantageous to a PON deployingmultiple types of ONUs in the same network, the innovative TDFM-PON ofthe present disclosure is fully applicable to a network using only onetype of ONU (e.g., only 100G ONUs in a multiple sub-band TFDM-PON). Sucha TDFM-PON would still realize significant improvements, in comparisonwith conventional PONs, in realizing more flexible TFDM bandwidthallocation.

More particularly, in comparison with conventional coherent PONs, thepresent coherent TFDM-PON system enables more flexible bandwidth sharingin both the time and frequency domains. Utilizing the improvedtunability and frequency selectivity of coherent optics, the presentembodiments advantageously enable the system to flexibly assign whichuser occupies which channels, to realize more effect load balancingand/or power saving capabilities. In exemplary operation, the improvedload balancing functionality of system 200 enables subscribers toutilize a particular channel at one time one channel, and a differentchannel at another time, such as in the case where traffic patterns mayprompt the subscriber to jump to another channel. Further to thisexemplary operation, even in the case of a subscriber transmitting usingall channels (e.g., 100G ONU 206(4), FIGS. 2-3), the present embodimentsenable the subscriber to transmit using only a single channel, or fewerthan all channels, to save power.

According the innovative features and flexibility of system 200, stillfurther improvements are realized over the conventional systems andtechniques, including without limitation, the ability to utilize, forsome or all of the different channels, different forward errorcorrection (FEC) techniques, different modulation formats, and differentMedia Access Control (MAC) layer functions. For example, in system 200,the respective sub-channels may be allocated such that the sub-channelsare independent of one another. Accordingly, each sub-channel mayutilize a different FEC to prevent the codeword from going acrosschannels. In this manner, system 200 is able to decode each of thechannels independently.

OLT Transmitter

FIG. 4 is a schematic illustration of a modulation scheme 400 for atransmitter 402 of OLT 202, FIG. 2. Transmitter 402 includes a lasersource 404, a modulator 406, and a processor 408 to output downstreamsignal 210. In general hardware configuration, transmitter 402 of thepresent coherent TFDM-PON is similar to that of a conventional TDM-basedcoherent PON. In particular, the bandwidth of both coherent transceiversmay be the same in operation. The signal generation processing oftransmitter 402 though, is significantly different from conventionaltechniques.

In the exemplary embodiment, processor 408 includes a data source 410, aserial-to-parallel converter 412, a first mapping unit 414, a secondmapping unit 416, a pulse shaping unit 418, and a digital-to-analogconverter (DAC) 420. In exemplary operation of processor 408, assumingthat system 200 utilizes N sub-bands, the data from data source 410 istransformed by serial-to-parallel converter 412 from a serial datastream to N parallel data streams. Each of N parallel data streams maythen be coded and mapped to QAM constellations by first mapping unit414, and then modulated and mapped to N sub-bands by second mapping unit416 corresponding to an N sub-band modulation spectrum 422. For eachsub-band 212, the modulation format may, for example, be for a singlecarrier modulation or multi-carrier modulations, such as orthogonalfrequency division multiplexing (OFDM) or discrete multi-tone (DMT).

In further exemplary operation, pulse shaping unit 418 applies sub-bandpulse shaping to an output of second mapping unit 416, that is, aftersub-band mapping, to reduce crosstalk between sub-bands. In anembodiment, pulse shaping unit 418 applies one or more pulse shapingdigital filters, including without limitation raised cosine filters forsquare root raised cosine filters. DAC 420 then combines the sub-bandsoutput from pulse shaping unit 418, and then converts the respectivesignals from the digital domain to analog domain for optical signalmodulation by modulator 406. In an embodiment, modulator 406 is a dualpolarization I/Q modulator. For illustration purposes, the several Nsub-bands 212 are shown to have substantially equal bandwidths to oneanother. Nevertheless, person of ordinary skill in the art willunderstand that one or more of the several sub-bands 212 may havedifferent bandwidths from other sub-bands without departing from thescope of the embodiments herein.

ONU Receivers

FIG. 5 is a schematic illustration of a single band detection and signalrecovery scheme 500 for a receiver 502 of ONUs 206, FIG. 2. Receiver 502includes an integrated coherent receiver (ICR) portion 504, a processor506, and an LO 508. In an exemplary embodiment, ICR portion 504 isconfigured as a single-band receiving unit capable of detecting onesub-band 212 of downstream signal 210, and processor 506 includes one ormore of an analog-to-digital converter (ADC) 510, a signal recovery unit512, a decision and decoding unit 514, and a data output portion 516.

In exemplary operation, a center frequency of LO 508 is tuned to thetarget sub-band 212 such that ICR portion 504 may coherently detect thetargeted signal. Once detected, the target signal is converted by ADC510, and then processed for signal recovery and decision by signalrecovery unit 512 and decision and decoding unit 514, respectively.Thus, through utilization of coherent frequency selectivity, which is aproperty unique to coherent detection, receiver 502 is advantageouslycapable of filtering out the target sub-band using only a passive filter(e.g., a low-pass filter, bandpass filter, etc.). In the embodimentdepicted in FIG. 5, since only one sub-band is detected, receiver 502may be configured for a detection bandwidth of only one sub-band. Inother words, in the case of a small-bandwidth subscriber needing todetect one sub-band, receiver 502 will be sufficiently configured withonly 1/N of the full receiver bandwidth (e.g., 100G).

Exemplary scheme 500 therefore illustrates the case for an ONU that onlydetects one sub-band. Nevertheless, as described above, the presentsystems and methods efficiently enable the coherent TFDM-PON to utilizeONU-side receivers of different types within the same network, that is,depending on the network structure. An exemplary embodiment for anONU-side receiver configured to detect multiple sub-bands is describedfurther below with respect to FIG. 6.

FIG. 6 is a schematic illustration of a multi-band detection and signalrecovery scheme 600 for a receiver 602 of ONUs 206, FIG. 2. Scheme 600is similar to scheme 500, FIG. 5, and includes an ICR portion 604, aprocessor 606, and an LO 608. Scheme 600 differs though, from scheme500, in that ICR portion 604 is configured as a multi-band receivingunit capable of detecting multiple sub-bands 212 of downstream signal210. Processor 606, similarly to processor 506, FIG. 5, includes an ADC610, a signal recovery unit 612, a decision and decoding unit 614, and adata output portion 616. Processor 606 differs from processor 506though, in that processor 606 additionally includes a sub-bandseparation unit 618 and a sub-band down conversion unit 620.Additionally, data output portion 616 is configured forparallel-to-serial conversion of the output data streams.

Accordingly, in the case of a large-bandwidth subscriber, scheme 600implements receiver 602 to be configured to have a large detectionbandwidth for detecting the multiple sub-bands. According to theadvantageous configuration of receiver 602, scheme 600 enables thedetection of the multiple sub-bands using only one ICR (i.e., ICRportion 604). Digital signal processing by processor 606 otherwiseoperates similarly to processor 506, except that after conversion by ADC610, sub-band separation and down conversion, by sub-band separationunit 618 and sub-band down conversion unit 620, respectively, may bedesired for parallel sub-band processing. According to scheme 600, foran ONU desiring to detect K sub-bands, the required bandwidth for signaldetection and data processing in receiver 602 need only be K/N of thefull bandwidth (100G, in this example).

OLT Receiver

In an exemplary embodiment, a receiver (not separately shown) for OLT202, FIG. 2, may be configured and operated similarly to ONU receiver602 and scheme 600. Nevertheless, because OLT 202 has the capability todetect all sub-bands, the corresponding OLT receiver is assumed to be afull bandwidth coherent receiver (i.e., K=N). However, in the case wheresome or many ONUs may share the same sub-band, an OLT receiver accordingto the present embodiments may further be configured to include a TDMburst receiver for multi-band coherent detection.

ONU Transmitters

As described above, the present coherent TFDM-PON may be configured toimplement multiple types of ONUs within the network using differentrespective subscribed bandwidths. Accordingly, for each different typeof ONU, the respective ONU-side transmitter in may also differ from oneanother. However, in the case where a particular ONU transmitter iscapable of generating multiple sub-bands the configuration and operationof such multi-band ONU transmitters will be similar to that oftransmitter 402 and scheme 400, FIG. 4. That is, the particularmulti-band ONU transmitter may generate two or more sub-bands for an ONUthat subscribes for more bandwidth. That is, for the generation of Ksub-bands, the required bandwidth for signal generation, processing, andmodulation at the ONU-side transmitter will be only K/N of the fullbandwidth.

Thus, the structure and operation of the ONU-side transmitter issubstantially similar to OLT-side transmitter 402, except that at theOLT-side, K=N. The structure and operation of the ONU-side transmittermay be different though, in the case where only a single sub-band isgenerated, as described further below with respect to FIG. 7.

FIG. 7 is a schematic illustration of a single sub-band modulationscheme 700 for a transmitter 702 of ONUs 206, FIG. 2. More particularly,scheme 700 illustrates a case where only one sub-band is used at theONU-side for signal modulation and mapping. Accordingly, transmitter 702is similar to transmitter 402, FIG. 4, and includes a laser source 704,a modulator 706, and a processor 708 to output and upstream signal 710.Processor 708 is also similar to processor 408, FIG. 4, and includes adata source 712, a first mapping unit 714 (e.g., for QAM mapping), asecond mapping unit 716 (e.g., for sub-band mapping), a pulse shapingunit 718, and a DAC 720. Different from processor 408, however,processor 708 requires no serial-to-parallel converter, which simplifiesboth the cost and operating requirements of transmitter 702. Transmitter702 otherwise operates similarly to transmitter 402, however requiredbandwidth for signal generation, processing, and modulation at ONUtransmitter 702 need only be K/N of the full bandwidth.

The embodiments described herein were modeled and simulated todemonstrate proof-of-concept, and the corresponding verification resultsare described further below with respect to FIGS. 8-13C.

FIG. 8 is a schematic illustration of an exemplary simulation model 800of a coherent TFDM-PON. More particularly, simulation model 800 isconfigured to verify operation of the exemplary TFDM-PON depicted inFIG. 2, accordingly, similar to system 200, FIG. 2, simulation model 800included a 100G central OLT transceiver 802, a splitter 804, and n ONUs806 (in the simulation model, n=4) in communication over an opticalfiber 808. System 800 differed, however, from system 200, in that ONUs806(1) and 806(2) were 50G transceivers, and ONUs 806(3) and ONU 806(n)were 100G transceivers to support a 100G coherent TFDM-PON transmissionsignal 810 having two sub-bands 812.

In operation of simulation model 800, the downstream receivers (notseparately shown) of the 50G ONUs (i.e., ONUs 806(1) and 806(2))detected one sub-band, and the corresponding ONU transmitters thereof(also not shown) generated only one sub-band each for the upstreamsignals. In contrast, the receivers of the 100G ONUs (i.e., ONUs 806(3)and 806(n)) were each able to detect two downstream sub-bands, and theircorresponding ONU transmitters were each able to generate two sub-bandsin the upstream. Allocation of LOs and frequency resources forsimulation model 800 are described further below with respect to FIG. 9.

FIG. 9 depicts an allocation scheme 900 for frequency resources and LOsof simulation model 800, FIG. 8. Scheme 900 thus demonstrates resultssimilar to scheme 300, FIG. 3, except that the allocation options inscheme 900 are fewer where only two different types of ONU transceiversare used in the network, as opposed to the four different typesdescribed above with respect to scheme 300. Scheme 900 thereforeillustrates the frequency resource and corresponding LO allocations inthe 100G coherent TFDM-PON of simulation model 800 with respect to thetwo downstream sub-bands 812(1), 812(2) using only 50G ONUs (i.e., ONUs806(1), 806(2)) and 100G ONUs (i.e., ONUs 806(3), 806(4)).

In operation of simulation model 800, scheme 900 demonstrates how thelower-speed 50G ONU cases were able to utilize either of the twosub-bands 812(1-2) as the allocated frequency resource, therebyproviding to different allocation options for both of 50G ONUs 806(1),806(2). In contrast, the 100G ONU cases each realized only one resourceallocation option (e.g., both of sub-bands 812(1-2)) for both of 100GONUs 806(3), 806(n). The person borders skill the art will understandthat even though the allocation options are fewer in scheme 900, theprinciples of scheme 300 still fully apply.

FIG. 10 is a graphical illustration depicting a comparative results plot1000 of downstream receiver sensitivity for ONUs 806, FIG. 8. Moreparticularly, plot 1000 represents the bit error rate (BER) againstreceived optical power (in dBm) for a first subplot 1002 of a 100Gsingle-band reference signal, a second subplot 1004 of a 100G signalaveraging both 50G sub-bands 812(1-2), a third subplot 1006 of 50G leftsub-band 812(1), and a fourth subplot 1008 of 50G sub-band 812(2).

In the embodiments depicted in FIGS. 10A-C, three different respectiveONU setups were tested: (1) a 100G ONU capable of detecting twosub-bands (i.e., subplot 1004); (2) one type of 50G ONU capable ofdetecting one sub-band on the left (i.e., subplot 1006); and (3) anothertype of 50G ONU capable of detecting one sub-band on the right (i.e.,subplot 1008). As a reference for comparison, the performance of 100Gsingle-band signal was also tested (i.e., subplot 1002). As can be seenfrom the example depicted in FIG. 10, no overt penalty is observed forthe different types of ONUs; all three types of ONUs demonstratesubstantially the same sensitivity performance in comparison with thereference signal.

FIGS. 11A-C are graphical illustrations depicting detected signalspectrum plots 1100, 1102, 1104 for ONUs 806, FIG. 8. More particularly,plot 1100 illustrates the detected downstream signal spectrum for the100G ONU with two sub-bands, plot 1102 illustrates the detecteddownstream signal spectrum for the 50G ONU with one sub-band on theright, and plot 1104 illustrates the detected downstream signal spectrumfor the 50G ONU with one sub-band on the left.

FIG. 12 is a graphical illustration depicting a comparative results plot1200 of receiver sensitivity for upstream detected signals from ONUs806, FIG. 8. More particularly, plot 1200 represents the BER againstreceived optical power at OLT 802 for a first subplot 1202 of the 50GONU that only modulates the upstream signal on the left sub-band, asecond subplot 1204 of the 50G ONU that only modulates the upstreamsignal on the right sub-band, and a third subplot 1206 of the 100Gupstream optical signals from two ONUs on both of the two sub-bands. Inthis example, equal power on two sub-bands may be assumed. As can beseen from the example depicted in FIG. 12, receiver sensitivity of asingle upstream sub-band realizes approximately 3-dB better performancethan in the case using both sub-bands in the upstream.

FIGS. 13A-C are graphical illustrations depicting respective detectedsignal spectrum plots 1300, 1302, 1304 for respective upstream detectedsignals 1204, 1202, 1206, FIG. 12. More particularly, plot 1300illustrates the detected upstream signal spectrum detected from the 50GONU on right sub-band, plot 1302 illustrates the detected stream signalspectrum from the 50G ONU on the left sub-band, and plot 1104illustrates the detected upstream signal spectrum from one of the 100GONUs on the two sub-bands.

Accordingly, it can be seen from the preceding embodiments that theinnovative coherent TFDM-PON solutions described herein effectivelymultiplex sub-bands in upstream and downstream transmissions, toefficiently utilize the advantageous properties of both time andfrequency division multiplexing in the same coherent PON, asdemonstrated by the simulation results described above.

Moreover, the present coherent TFDM-PON systems and methods support bothasymmetric ONU/OLT hardware setups, and also a desirable pay-as-you-goONU cost capability based on the particular bandwidth subscription. Thepresent techniques therefore demonstrate a significant improvement overconventional WDM-PON or TWDM-PON systems. For example, in the TFDM-PONof the present embodiments, all the ONUs and OLT may be configured towork at the same wavelength grid with a small frequency tuning. Theutilization of coherent detection wavelength selective features thuseliminates the need for optical filtering or wavelength selectivecomponents for frequency selection.

The innovative coherent TFDM-PON of the present application thereforedemonstrates still further advantages over the conventional techniquesby providing more flexible bandwidth sharing with respect to both timeand frequency. Furthermore, the present coherent TFDM-PON is fullycompatible to existing TDM-PON system, and may operate using the powersplitter-based ODN of such conventional architectures without colorfulcomponents. Through implementation of frequency division multiplexing,the overall scheduling latency in the present coherent TFDM-PON is alsoexpected to be lower than it would be using conventional techniques.

According to the innovative systems and methods described herein,effective architectures and operation principles for a coherent TFDM-PONare provided for improved bandwidth allocation flexibility, but at alower hardware and operational cost. Additionally, several innovativetransmitter and receiver designs configurations are presented for bothof the OLT-side and ONU-side transceivers, and with respect to bothhardware configurations and digital processes of the relevant softwaremodules of the processor. The modeling and verification resultsdescribed above provide proof-of-concept for all of these innovativeprinciples.

Exemplary embodiments of optical communication systems and methods aredescribed above in detail. The systems and methods of this disclosurethough, are not limited to only the specific embodiments describedherein, but rather, the components and/or steps of their implementationmay be utilized independently and separately from other componentsand/or steps described herein. Additionally, the exemplary embodimentscan be implemented and utilized in connection with other access networksutilizing fiber and coaxial transmission at the end user stage.

The present embodiments are particularly useful for networks andcommunication systems implementing a DOCSIS protocol, and may thereforebe advantageously configured for use in existing 4G and 5G networks, andalso for networks compatible with new radio (e.g., 5G-NR) and futuregeneration (e.g., 6G) network implementations. The present systems andmethods may therefore be implemented with respect to DOCSIS protocols,as well as other protocols, including without limitation EPON, RFoG,GPON, and/or Satellite Internet Protocol, without departing from thescope of the embodiments herein.

Although specific features of various embodiments of the disclosure maybe shown in some drawings and not in others, such illustrativetechniques are for convenience only. In accordance with the principlesof the disclosure, a particular feature shown in a drawing may bereferenced and/or claimed in combination with features of the otherdrawings.

Some embodiments involve the use of one or more electronic or computingdevices. Such devices typically include a processor or controller, suchas a general purpose central processing unit (CPU), a graphicsprocessing unit (GPU), a microcontroller, a reduced instruction setcomputer (RISC) processor, an application specific integrated circuit(ASIC), a programmable logic circuit (PLC), a field programmable gatearray (FPGA), a digital signal processor (DSP) device, and/or any othercircuit or processor capable of executing the functions describedherein. The processes described herein may be encoded as executableinstructions embodied in a computer readable medium, including, withoutlimitation, a storage device and/or a memory device. Such instructions,when executed by a processor, cause the processor to perform at least aportion of the methods described herein. The above examples areexemplary only, and thus are not intended to limit in any way thedefinition and/or meaning of the term “processor.”

This written description uses examples to disclose the embodiments,including the best mode, and also enables a person skilled in the art topractice the embodiments, including the make and use of any devices orsystems and the performance of any incorporated methods. The patentablescope of the disclosure is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal language of the claims.

What is claimed is:
 1. A coherent optical transmitter, comprising: alaser source; a data source including a data stream; a processorincluding (i) a serial-to-parallel converter for converting the datastream into a plurality of parallel data streams, (ii) a modulationmapping unit configured to code and map each of the plurality ofparallel data streams to a constellation of a modulation format togenerate a plurality of modulated data streams, (iii) a sub-band mappingunit configured to map each of the plurality of modulated data streamsto a respective sub-band modulation spectrum from a plurality offrequency sub-bands to generate a plurality of sub-band data signals,and (iv) a digital-to-analog converter configured to combine theplurality of sub-band data signals and generate a combined analogsub-band output of processed data; and a modulator configured tomodulate the combined analog sub-band output of processed data with thelaser source to generate a modulated optical signal for transmissionover an optical transport medium.
 2. The transmitter of claim 1,comprising an optical line terminal (OLT).
 3. The transmitter of claim2, wherein the OLT comprises a 100G OLT.
 4. The transmitter of claim 1,comprising an optical network unit (ONU).
 5. The transmitter of claim 4,wherein the ONU comprises a 100G transceiver.
 6. The transmitter ofclaim 1, wherein the processor is further configured to dynamicallyallocate first and second sub-bands of the respective sub-bandmodulation spectrum to first and second remote transceivers disposedopposite the transmitter with respect to the optical transport medium.7. The transmitter of claim 6, wherein the first remote transceiverincludes a first local oscillator (LO) having a first LO centerfrequency tuned to the first sub-band, and wherein the second remotetransceiver includes a second LO having a second LO center frequencytuned to the second sub-band.
 8. The transmitter of claim 7, comprisinga 100G coherent transmitter, and wherein the first remote transceiverincludes one of a 25G transceiver, a 50G transceiver, and a 75Gtransceiver.
 9. The transmitter of claim 8, wherein the second remotetransceiver includes one of a 100G transceiver.
 10. The transmitter ofclaim 6, wherein the first sub-band comprises a single carriermodulation format.
 11. The transmitter of claim 10, wherein the secondsub-band comprises a multi-carrier modulation format.
 12. Thetransmitter of claim 11, wherein the multi-carrier modulation formatcomprises at least one of orthogonal frequency division multiplexing anddiscrete multi-tone.
 13. The transmitter of claim 6, configured forpoint-to-multipoint communication to the first and second transceivers.14. The transmitter of claim 1, wherein two or more sub-bands of theplurality of sub-bands have an equal bandwidth.
 15. The transmitter ofclaim 1, wherein two or more sub-bands of the plurality of sub-bandshave different bandwidths.
 16. The transmitter of claim 1, wherein theplurality of sub-bands are distributed about a base frequencycorresponding to a wavelength grid for the downstream optical signal.17. A transmitter for a coherent passive optical network (PON),comprising: (i) a laser source; (ii) a downstream processor configuredto (i) map a downstream data stream to a plurality of sub-bands of afrequency spectrum for a downstream optical signal modulated with theplurality of sub-bands, and (ii) dynamically allocate first and secondsub-bands of the plurality of sub-bands to first and second upstreamtransceivers, respectively, disposed remotely from the transmitter overan optical transport medium of the PON; (iii) a modulator configured tomodulate the dynamically allocated first and second sub-bands with thelaser source to generate a modulated optical signal as the downstreamoptical signal for transmission to the first and second upstreamtransceivers over the optical transport medium; and (iv) aserial-to-parallel converter for converting the data stream into aplurality of parallel data streams; (v) a modulation mapping unitconfigured to code and map each of the plurality of parallel datastreams to a constellation of a modulation format to generate aplurality of modulated data streams; (vi) a sub-band mapping unitconfigured to map each of the plurality of modulated data streams to arespective sub-band modulation spectrum from the plurality of sub-bandsto generate respective plurality of sub-band data signals; and (vii) adigital-to-analog converter configured to combine the respectiveplurality of sub-band data signals and generate a combined analogsub-band output of processed data.
 18. A transmitter for a coherentpassive optical network (PON), comprising: a laser source; a downstreamprocessor configured to (i) map a downstream data stream to a pluralityof sub-bands of a frequency spectrum for a downstream optical signalmodulated with the plurality of sub-bands, and (ii) dynamically allocatefirst and second sub-bands of the plurality of sub-bands to first andsecond upstream transceivers, respectively, disposed remotely from thetransmitter over an optical transport medium of the PON; and a modulatorconfigured to modulate the dynamically allocated first and secondsub-bands with the laser source to generate a modulated optical signalas the downstream optical signal for transmission to the first andsecond upstream transceivers over the optical transport medium, whereinthe first and second sub-bands have different respective modulationformats.